There has been known a technology in which an NOx storage reduction catalyst (hereinafter referred to as an NSR catalyst) is arranged in an exhaust passage of an internal combustion engine. This NSR catalyst serves to occlude or store NOx contained in an incoming exhaust gas when the oxygen concentration of the incoming exhaust gas is high, and to reduce the occluded or stored NOx when the oxygen concentration of the incoming exhaust gas becomes low and when a reducing agent exists. In order to decrease the oxygen concentration of the exhaust gas flowing into the NSR catalyst as well as to supply the reducing agent to the NSR catalyst, rich spike control is carried out in which the internal combustion engine is driven to operate temporarily at an air fuel ratio equal to or less than a stoichiometric air fuel ratio.
In addition, there has also been known a technique in which when the NOx stored by the NSR catalyst is reduced, the concentration of oxygen in the exhaust gas is decreased by increasing an amount of fuel injection so as to adjust the air fuel ratio to a value in the vicinity of the stoichiometric air fuel ratio, after enhancing an EGR (exhaust gas recirculation) ratio to a predetermined value (for example, refer to a first patent literature). In this technology, the variation of the air fuel ratio in a combustion chamber is suppressed by taking into consideration a response delay of an EGR gas.
Moreover, there has been known a technology in which in cases where a determination is made that there is an ignition delay, when the air fuel ratio of the exhaust gas is richer than the stoichiometric air fuel ratio, the ignition delay is suppressed by controlling the EGR ratio to about 15% (for example, refer to a second patent literature).
Further, there has also been known a technology in which when a required amount of injection has increased, an actual amount of injection is made to increase gradually, and at the same time an EGR valve is made to open gradually (for example, refer to a third patent literature).
Furthermore, there has also been known a technology in which when post injection is carried out, a target air fuel ratio for feedback control is decreased and an amount of recirculation of the exhaust gas is increased, whereas in a predetermined period of time from the start of the execution of this post injection, an amount of operation for feedback is made small (for example, refer to a fourth patent literature).
However, when rich spike control is carried out in an internal combustion engine which is operated at a lean air fuel ratio, the combustion temperature becomes high due to an increase in the amount of fuel, so the amount of discharge of NOx may increase. That is, in spite of reducing the NOx stored in the NSR catalyst, the amount of NOx discharged from the internal combustion engine may increase. For this reason, there is a fear that NOx may not be treated fully by the NSR catalyst, and so NOx may pass through the NSR catalyst. Even if the EGR ratio has been enhanced to the predetermined value as in the past, there will be a fear that NOx may not be decreased to a sufficient extent.